The present invention relates to an injection-molding apparatus for molding a multi-layered article and a method of injection-molding a multi-layered article.
In recent years, containers formed of thermoplastic resins are widely used as containers for cosmetics, foods, beverages and the like. The development of containers formed of a polyethylene terephthalate resin (to be sometimes abbreviated "PET resin", hereinafter) in particular is rapidly advancing owing to improvements in biaxial orientation blow molding technology. However, a container formed of a biaxially oriented thermoplastic polyester resin composed mainly of PET resin does not necessarily have complete performances. That is, when the content to be filled in a container is a food which requires high gas barrier properties against oxygen gas and carbon dioxide gas, and therefore, the container formed of PET resin has a defect that the taste of the contents is impaired. With recent downsizing of containers, the containers are increasingly required to have higher gas barrier properties.
For satisfying the above requirement, JP-A-57-128520 (corresponding to U.S. Pat. No. 4,535,901) discloses a method in which an injection-molding apparatus having two injection cylinders for separately plasticizing and melting a thermoplastic polyester resin and an m-xylylene-group-containing polyamide resin (to be sometimes abbreviated as "MX nylon resin", hereinafter) which is a thermoplastic gas-barrier resin, respectively, and having a single mold is used, and in one molding cycle, the molten thermoplastic polyester resin is injected, the molten MX nylon resin is injected while the molten thermoplastic polyester resin is being injected, and the injection of the molten thermoplastic polyester resin is continued even after completion of injection of the molten MX nylon resin, to form a comparison having a three-layer structure of the thermoplastic polyester resin layer/the MX nylon resin layer/the thermoplastic polyester resin layer. The above injection molding method is called a simultaneous-injection molding method.
Further, JP-A-60-240409 (corresponding to EP No. 161625/1985) discloses an injection-molding method similar to the above method, in which, for example, a molten thermoplastic polyester resin and a molten MX nylon resin are injected under specific conditions in the order of the molten thermoplastic resin, the molten MX nylon resin and the thermoplastic polyester resin, to form a parison having a five-layer structure of the thermoplastic polyester resin layer/the MX nylon resin layer/the thermoplastic polyester resin layer/the MX nylon resin layer/the thermoplastic polyester resin layer. The above method is called an alternate-injection molding method.
In recent years, further, a technique is being developed for pelletizing collected containers composed mainly of PET resin and recycling them as a recycled PET resin. When the above recycled PET resin (including a PET resin reclaimed from defective products in the production of parisons and a recycled PET resin once used in the production of parisons) is used to produce parisons for beverage containers, it is required to form a parison having a three-layer structure of the PET resin layer/the recycled PET resin layer/the PET resin layer or a five-layer structure of the PET resin layer/the recycled PET resin layer/the PET resin layer/the recycled PET resin layer/the PET resin layer.
The methods disclosed in the above Japanese Laid-open Patent Publications have enabled the production of multi-layered containers (multi-layered bottles) having an appearance and mechanical performances equivalent to those of a container formed of a polyethylene terephthalate resin and having greatly improved barrier properties against oxygen gas and carbon dioxide gas, and thus-produced containers are now being used.
Conventionally, when a multi-layered molded article (e.g., a multi-layered parison as a precursor of a multi-layered container) formed of two or more resins is produced by virtue of two or more injection cylinders, there is used an injection molding apparatus having a mold provided with a cavity and having two or more injection cylinders. For example, in an injection molding apparatus having two injection cylinders, resin flow-passages (a first resin-flow-passage in which a first molten resin flows and a second resin-flow-passage in which a second molten resin flows) connecting the insides of the injection cylinders to the cavity are structured so as to meet with each other in a junction portion upstream of a gate portion opened to the cavity. Portions of the first and second resin-flow-passages positioned within the mold have a hot runner structure. Further, portions of the first and second resin-flow-passages upstream of the junction portion generally have the structure of a multiple tube. A screw provided in each injection cylinder is generally structured so as to move back when a pressure is exerted from the resin-flow-passage in any case other than the cases of injection of the molten resin and application of a dwell pressure, so that the molten resin in each resin-flow-passage flows back into each injection cylinder.
In the injection molding apparatus having the above structure, when a first molten resin for forming the outermost layer of the multi-layered article is injected into the cavity through the first resin-flow-passage for completely filling the cavity with the first and second molten resins, the first molten resin flowing in the first resin-flow-passage flows into the second resin flow-passage. In this case, the inflow of the first molten resin is not constant. As a result, the amount of the first molten resin to be injected into the cavity is destabilized. Further, during the first injection of the first molten resin, the second molten resin present in the second resin-flow-passage near the junction portion and the first molten resin are injected into the cavity together in a state of a mixture of these resins. There is therefore caused a problem that the second resin which is not to compose the outermost layer of the multi-layered article comes to exist on the outermost layer surface.
For preventing the above flow of the first molten resin into the second resin-flow-passage, a prior art uses a hydraulic shut-off valve provided in a nozzle portion of the injection cylinder which is for injecting second molten resin. only during the injection of the second molten resin, the shut-off valve is opened, and in any other case, the shut-off valve is closed, whereby the flowing of the first molten resin into the second resin-flow-passage is prevented.
When the flow of a molten resin is controlled by means of the shut-off valve, the flowing of the first molten resin into the second resin-flow-passage can be prevented. However, there is caused the following problem. During the first injection of a first molten resin, the first molten resin and a second molten resin are mixed in the vicinity of the junction portion of the resin-flow-passages, or a second molten resin which is not in an injected state is taken into the first molten resin which is being injected. As a result, the second molten resin present near the junction portion flows into the cavity, and the second resin comes to exist on the surface of the multi-layered article. That is, there is caused a problem that the second resin which is not to compose the outermost layer of the multi-layered article comes to exist on the outermost layer surface.
JP-A-61-206612 (corresponding to U.S. Pat. No. 4,657,496) discloses a hot liner mold for injection-molding, which has a first resin-flow-passage 11 and a second resin-flow-passage 12, the second resin-flow-passage 12 being provided with a check valve 13. It is said that when a first resin material A is injected, a cavity 22 is completely filled with the resin but that since the second resin-flow-passage 12 is provided with the check valve 13, a second resin material B flows back due to the first resin material A in no case.